The present invention relates to an apparatus for blade coating of a travelling web, preferably a paper web, and the invention also covers a process for providing such coating.
In devices for blade coating of travelling webs, in particular paper webs, the application of the coating composition is controlled by the engagement pressure of the doctor blade against the web, the pressure per unit of surface at the contact area of the blade against the web being substantially determining of the applied quantity of coating composition. Since the pressure per unit of surface is decisive for the applied quantity it is thus essential that the engagement surface of the blade is parallel in relation to the web.
In known processes for such coating the blade is clamped in a blade holder, the blade being bent or curved as seen in a cross section in a plane extending perpendicular to the web and in the direction of its travel. By this bending of the blade there is obtained a spring force acting against the paper web. The most common and easiest way of affecting the blade pressure, i.e. to change the spring force, is to vary the distance between the blade attachment site and the travelling web. In this process the bending of the blade is affected, which also causes a change of the spring force of the blade against the web. The disadvantage of this process is that the parallelism between the edge of the blade and the web as indicated above will be lost, the applied quantity of coating composition being affected in a non-desired way up to the moment when the edge of the blade has been abraded to renewed parallelism with the web.
In order to avoid this inconvenience in the conventional art methods have been developed for the purpose of controlling pressure without changing the angle of engagement of the blade. This has been done by bending the blade in such a manner that the spring force will be changed without significant change of the angle of engagement of the blade against the web. Such a process is described in Swedish patent 8502927-0. The development of paper machines has resulted in processing of web width of up to 8 to 10 m. This development has resulted in great difficulties in connection with the use of presently available techniques, in view of the fact that with such width of the web it is difficult to manufacture a blade holder which will not be deformed, i.e. bent, under the influence of varying forces originating from blade pressure, heat tensions, deflection by inherent weight, etc. Since the doctor blade in the known technique is rigidly clamped in the blade holder the bending or deformation of the latter will affect both the angle of engagement of the blade and the pressure in its engagement against the web in a non-desired manner.
For the purpose of avoiding these problems methods have been developed for the purpose of enabling continuous measurement of the rectilinearity of the blade holder and automatic compensating for deviations from rectilinearity while using hydrolic systems. The disadvantage of such technical solution is the fact that it is technically much too advanced and results in high investment costs and, moreover, it requires extensive maintenance. Techniques are also developed whereby it is possible to point-wise, automatically or manually, exert pressure on the blade in order to compensate for the variations arising at the area of a the blade engagement when the blade holder is deformed. The disadvantage associated with these techniques consists in the fact that by exerting a varying pressure on the blade the parallelism between the surface of engagement of the blade and the web will be affected which has for an effect that these techniques are technically very difficult to apply.